There are more than 80 types of human papillomavirus (HPV), many of which have been associated with a wide variety of biological phenotypes, from benign proliferative warts to malignant carcinomas (for review, see McMurray et al., Int. J. Exp. Pathol. 82(1): 15-33 (2001)). HPV6 and HPV11 are the types most commonly associated with benign warts and/or nonmalignant condyloma acuminata of the genital or respiratory mucosa. HPV16 and HPV18 are the high-risk types most frequently associated with in situ and invasive carcinomas of the cervix, vagina, vulva and anal canal. More than 90% of cervical carcinomas are associated with infections of HPV16, HPV18 or the less prevalent oncogenic types HPV31, -33, -45, -52 and -58 (Schiffman et al., J. Natl. Cancer Inst. 85(12): 958-64 (1993)). The observation that HPV DNA is detected in more than 90% of cervical cancers provides strong epidemiological evidence that HPVs cause cervical carcinoma (see Bosch et al., J. Natl. Cancer Inst. 87(11): 796-802 (1995)).
Papillomaviruses are small (50-60 nm), nonenveloped, icosahedral DNA viruses that encode up to eight early and two late genes. The open reading frames (ORFs) of the viral genomes are designated E1 to E7, and L1 and L2, where “E” denotes early and “L” denotes late. L1 and L2 code for virus capsid proteins, while the E genes are associated with functions such as viral replication and cellular transformation.
The L1 protein is the major capsid protein and has a molecular weight of 55-60 kDa. The L2 protein is the minor capsid protein. Immunological data suggest that most of the L2 protein is internal to the L1 protein in the viral capsid. Both the L1 and L2 proteins are highly conserved among different papillomaviruses.
Expression of the L1 protein or a combination of the L1 and L2 proteins in yeast, insect cells, mammalian cells or bacteria leads to self-assembly of virus-like particles (VLPs) (for review, see Schiller and Roden, in Papillomavirus Reviews: Current Research on Papillomaviruses; Lacey, ed. Leeds, UK: Leeds Medical Information, pp 101-12 (1996)). VLPs are morphologically similar to authentic virions and are capable of inducing high titres of neutralizing antibodies upon administration into an animal. Because VLPs do not contain the potentially oncogenic viral genome, they present a safe alternative to the use of live virus in HPV vaccine development (for review, see Schiller and Hidesheim, J. Clin. Virol. 19: 67-74 (2000)). For this reason, the L1 and L2 genes have been identified as immunological targets for the development of prophylactic and therapeutic vaccines for HPV infection and disease.
HPV vaccine development and commercialization have been hindered by difficulties associated with obtaining high expression levels of exogenous genes in successfully transformed host organisms, limiting the production of purified protein. Therefore, despite the identification of wild-type nucleotide sequences encoding HPV L1 proteins such as HPV45 L1 protein, it would be highly desirable to develop a readily renewable source of crude HPV L1 protein that utilizes HPV45 L1-encoding nucleotide sequences that are optimized for expression in the intended host cell. Additionally, it would be useful to produce large quantities of HPV45 L1 VLPs having the immunity-conferring properties of the native proteins for use in vaccine development.